PCB design has changed throughout my career, and many of the tasks that I used to have to do to complete the job would practically be considered a crime today. For instance, who out there manually edits the netlist these days to add testpoint components and their net assignments? If you are, you may want to consider upgrading your PCB design CAD system. One thing that used to be a crime, (and still is in my opinion), was how some layout designers considered their involvement with the design to be over when trace routing was complete.
Even though the circuit board layout may be completely placed and routed, it cannot be built until all of the manufacturing data is prepared, collected, and sent out to the manufacturer. This means that the layout team needs to continue their work until they have successfully handed the design off to the PCB contract manufacturer for the fabrication and assembly of the board. To finish our four-part series on the PCB design process, we are going to look at what is needed from the layout team to successfully bring the project to PCB design completion.
Before You Consider PCB Design Completion, Run the DRCs!
In the first three parts of this series we have looked at the schematic capture process, placing components in the physical layout of the board, and finally routing the net connections of the layout. Now it is time to get the board ready to be manufactured, but before we do that there is one critical step that has to be done; running the design rule checks (DRCs).
DRCs are built into the CAD systems to make sure that the proper spacing width and other design constraints are obeyed in the layout. These rules can be set up to specify rules like trace widths or space clearances for one object, such as a net or a component, or a group of objects. The advanced versions of these rules can also be set up to govern the length and topology patterns of the trace routing for high-speed or other unique design constraints. Many tools offer 3D viewing and check the layout against imported mechanical data to verify the PCB clearances to other portions of the overall system.
Online DRCs normally run in the background while you design, but it’s a good design practice to run the rules again in a batch mode after the design is completed. There are many scenarios where an online DRC warning was missed during layout, or an update to a part changed the configuration of the board. It is essential therefore that the DRCs be run one last time, and that any warnings are accounted for before the design is released for manufacturing.
Another important step before the completed design can be released for manufacturing is to conduct a review of the design with all personnel involved. This would include mechanical, electrical, test, and manufacturing engineering as well as anyone else that has had input in the design process. Even in smaller companies with limited staff, it is still a good idea to get another pair of eyes on the design before you commit to building boards. Once the design has been cleared through these processes it can be prepared for manufacturing.
The Final Six Steps to Ensure the Completion of the PCB Design
Here are the six areas that a PCB designer will need to focus on to release the design for fabrication and assembly:
- Testpoints: To verify the connectivity and component assembly of the PCBs, the manufacturers will conduct different types of automated testing. These tests require that a small metal area on each net (called a testpoint) be identified as a location that a probe tip can access. The PCB design software usually provides the capabilities to flag pins, vias, or other metal areas as testpoints so that those locations can be used to build the test probe fixtures. If adding testpoints require altering any of the previously laid out placements or routing of the board, the DRCs will have to be run again.
- Renumber: Each component on the board will have a unique reference designator number assigned to it from the schematic. For technicians to easily find parts on the board, it’s a common practice to renumber these reference designators. This can be done either sequentially, or according to a grid pattern using specialized features in the CAD software.
- Synchronization: With the component reference designators renumbered—plus any pin or gate swapping that might have been done during design—the layout will no longer match the schematic. To remedy this, the databases must be synchronized together again. Not only will this update the schematic, but if any parts have been updated or changed on the schematic, that information will be passed on to the layout as well.
- Silkscreen: All of the component reference designators will be printed onto the board using a silkscreen process from files generated from the CAD database. This requires rotating, maneuvering, and even changing the size of the designators in the CAD database to make them all readable. At the same time, other PCB markings like part and revisions numbers, and company contact and legal information will be added.
- Drawings: To build and assemble the printed circuit board, fabrication and assembly drawings need to be created to serve as manufacturing instructions. The fabrication drawing will dimension the board’s outline features and show the locations of the drilled holes, as well as list the different tolerances and specifications. The assembly drawing will show the position of the components on the board along with any specific instructions for their installation.
- Manufacturing Files: All of the data is now ready to be generated from the CAD system into files that can be delivered to the contract manufacturer. Some systems can output this data in a batch file, while others will generate individual files that the designer will have to bundle together. Whichever way that is chosen, the files that are generated normally include the following:
- Gerbers: Image files of the different PCB design layers.
- NC Drill: Locations for all the drilled holes in the board.
- Bill of Materials (BOM): List of all components used on the board.
- Pick and Place: Locations of all components for automated placement.
- Testpoint data: Locations of all testpoints for probe access.
- Netlist: List of all the nets connecting the component pins.
- Drawings: The fabrication and assembly drawings, usually in a PDF format.
- Readme Files: Unique manufacturing instructions that are not found in other documents.
This completion list can change from board to board as other tasks may be required depending on the needs of each PCB design. To ensure that your design is completed and ready for fabrication and assembly, it’s a good idea to work ahead with your PCB contract manufacturer.
Plan Ahead with Your PCB CM for Manufacturing Files
Not only may the unique aspects of each PCB design require different manufacturing files and formats, but each CM may have requirements as well. As such, it’s advantageous to work ahead with your manufacturer to find out their needs and plan accordingly. Additionally, CMs can work with your data in case changes are needed to make the board more manufacturable. If that’s the case, you’ll need to be ready to receive any changed data back from them to incorporate into the archives of your PCB design.
At VSE we have been working with different PCB designs over the years, and have a wide range of expertise in building various circuit board technologies. We understand the challenges that you face as a PCB designer, and our staff of engineers and technicians are ready to work together with you to efficiently transition your design from CAD to manufacturing.